I have kept accurate records for years of my fuel usage. I have done many comparisons with premium fuels. Some years ago when they first came on the market, I think about 1994, they were between 2 and 4c a litre dearer. I have looked up some records and back in 2006 when unleaded was running around anywhere between 135 to 145c a litre premium, or in Caltex's case, Vortex was around 8c a litre dearer. Over an extended trial period I discovered that factualy I could say that I was getting about 4c a litre better economy but not 8c. (2003 Magna AWD sports) I believe I got marginally better HP but that is not backed up by any definitive test. However the bottom line was yes, I gained 4c a litre worth but it cost me 8c a litre to do it. Now on the point of keeping your engine cleaner for longer and improve longevity, who is ever going to easily proove that point. Not many people keep their cars until some many thousand of kilometres later, the engine fails and can attribute better fuel to its longer life. The other thing today is 'premium',fuels are much dearer so we are still being conned.
As far as diesel, I now have a diesel Navara and have being buying Vortex because that is all that is sold at the local garage in Wingham NSW. (I also have a caltex fuel card) And it is also cheaper then standard diesel at the nearest pump in Taree. Fuel tests are inconclusive and although I may well keep this ute to the death I don't now I'll ever be able to say conclusively that its exteneded engine life was due to using premium fuels.